Broadband technologies are taking a predominant role in the emerging information society, and, in particular, broadband satellite communication systems are being broadly employed to respond to the growing requirements of the information society. More specifically, based on global access and broadcasting capabilities, satellite communication systems are well suited to provide broadband services to remote locations and highly mobile users (e.g., broadband services provided to rural areas and to ships, aircraft, trains, etc.), as well as to major metropolitan areas of high population density and high broadband demands. Accordingly, the overall demand for broadband capacity continues to increase exponentially. Bandwidth availability limitations of satellite systems, however, continues to be a predominant issue in the continued growth of this communications technology.
In order to satisfy the growth in demand for high availability broadband capacity, broadband satellite communications systems that deploy high throughput satellites are becoming more prevalent. High throughput satellite (HTS) is a classification for a communications satellite that provides upwards of more than 20 times the total throughput of a classic FSS geostationary communications satellite (e.g., throughputs of more than 100 Gbit/sec of capacity are currently being deployed, which amounts to more than 100 times the capacity of a conventional Ku-band satellite). Moreover, these satellites typically utilize the same amount of allocated orbital spectrum, and thus significantly reducing cost-per-bit. The significant increase in capacity of an HTS system is achieved by employing wideband satellite technology, including an increased number of beams of a given satellite to increase the available bandwidth and thereby increase the respective capacity of the satellite.
Multi-beam communications satellites (e.g., spotbeam satellites) are generally designed such that a given geographic coverage area is serviced by a pattern of beams generated by a phased array antenna, where the individual beams and associated beam pattern are formed via a beamforming network deployed either onboard the satellite or deployed at a ground-based network control center. Further, with such multi-beam satellites, in order to avoid or minimize inter-beam interference, certain frequency reuse principles must be applied to the beam patterns of the antenna design. One of the primary guidelines for the beam pattern or frequency reuse pattern is that a frequency and polarization combination of one beam cannot be “reused” within a certain distance from another beam of the same frequency and polarization combination. The distance between beams is generally specified as the distance between beam centers of two beams of a same color (two beams of the same frequency band and polarization), where the distance is quantified in terms of the radius r of the beams. If the minimum distance requirements are not followed with regard to two such beams, then the beams will cause unacceptable levels of interference between them. For example, a reuse of four means that a set of four adjacent beams will have disjoint frequency and polarization assignments such that none of the beams of each set interfere with each other. In other words, adjacent sets of four beams separate the beams sharing a common frequency and polarization such that (even though they are reusing the same frequency and polarization assignments) the beams of one set will not excessively interfere with the respective beams of an adjacent set. With high-throughput multibeam satellites, a high level frequency reuse and spotbeam technology is employed to enable frequency reuse across multiple narrowly focused spotbeams (usually in the order of 100's of kilometers). Further, in order to provide an adaptive distribution of capacity the coverage area of the satellite (e.g., to address a non-uniform distribution of users and capacity demand over the coverage area), satellite architectures may employ adaptively or dynamically steerable beams.
A phased array antenna generally comprises multiple radiating elements arranged in an array format that are electrically scanned to generate desired beam pattern of radio waves that can be electronically steered to point in different directions without physically moving the antenna or antenna elements. The individual beams are formed through the shifting of the phase and amplitude of the signal emitted from each radiating element, which serves as constructive interference toward the desired direction for the waves and as destructive interference for undesired directions. The main beam in a phased array antenna points in the direction of the increased phase shift. Adding a phase shift to the signal received or transmitted by each antenna in the array results in the collective signal of the individual antenna elements to act as the signal of a single antenna with performance vastly different from the individual antennas in the array. In an array antenna, the radio frequency current from the transmitter is fed to the individual antennas with the correct phase relationship so that the radio waves from the separate antennas add together to increase the radiation in a desired direction, while cancelling to suppress radiation in undesired directions. In a phased array, the power from the transmitter is fed to the antennas through individual amplifiers and phase shifters that electronically alter the amplitude and phase of the output of each antenna element thereby steering the beam(s) of the antenna. In other words, both the amplitude and phase of each antenna element are controlled, where a combined amplitude and phase control are used to adjust side lobe levels and steer the resulting beams. A combined relative amplitude ak and phase shift ϕk for each antenna element k is applied via a respective complex weight wk applied to the signal for the respective antenna element. In a multibeam system, a matrix of complex weights is applied to the antenna feed or element signals to generate the desired beams.
As provided above, the beamforming may be employed as a space-based approach where the beamforming network is implemented in the satellite payload, or as a ground-based approach where the beamforming network consists of a ground-based implementation, for example deployed at one or more gateway locations. In the case of a ground-based beamforming approach, for the forward link (the link from the gateway to the user terminals (UTs), the gateway generates the individual signals for the downlink of each feed element of the satellite antenna.
FIG. 1A illustrates a traditional beamforming architecture for the forward-link in a multibeam wireless communications system. For a multi-beam forward link transmission, at the gateway, the transmission signal for each respective satellite downlink beam is fed into a beamformer, which applies the amplitude and phase weighting via a number of complex multiplications based on an array of complex weights to generate the feed signals for respective transmission by the elements of the satellite antenna array. For example (as shown in FIG. 1A), for M beams and N antenna elements, complex multiplications are applied to the M beam signals (S1, S2, S3, . . . , SM) based on an N×M beamforming matrix of complex weights to generate the N element signals (e1, e2, . . . , eN). The N element signals are then transmitted to the satellite. The satellite processes and provides each of the received element signals to the respective antenna element, and the antenna transmits the resulting M beams in a beam pattern based on the complex amplitude and phase weights applied by the gateway beamforming network.
FIG. 1B illustrates a traditional beamforming architecture for the return-link in a multibeam wireless communications system. In the return direction, the UTs transmit uplink signals to the satellite, which are received by the satellite in the respective beams within which each of the UTs is located. The satellite in turn transmits each of the feed or element signals (as received by the respective elements of the satellite antenna) to the gateway. At the gateway, the received element signals are fed into a beamformer, which similarly applies amplitude and phase weighting via a number of complex multiplications based on an array of complex weights to regenerate the individual beams as received by the satellite. For example, for the M beam and N antenna element example, complex multiplications are applied to the N element signals (e1, e2, . . . , eN) based on an M×N matrix of complex weights to generate the M beam signals (S1, S2, S3, . . . , SM). The resulting M beam signals (S1, S2, S3, . . . , SM) are then processed by the gateway to decode the original transmissions of the respective UTs.
In high-throughput satellite systems, however, as bandwidth, adaptability and flexibility requirements increase, the increased bandwidth and increased number of narrow beams results in a higher required rate of complex multiplications by the beamforming network, which introduces added complexity and design challenges.
What is needed, therefore, is an approach for adaptive beamforming for a multibeam wireless communications system that simplifies the required beamforming computations and associated complexity of the beamforming network without sacrificing the number of achievable beams and the system throughput.